1. Field of the Invention
This invention relates to particle sources. In particular the invention relates to particle sources for producing short bursts of particles.
Such sources are used, for example, in a time-of-flight mass spectrometer to produce bursts of charged or neutral particles which in turn are directed onto a sample so as to excite bursts of ions from the sample, the bursts being of typically one to one hundred nanoseconds duration. The times for the bursts of ions from the sample to travel a certain distance are measured. As these times are dependent on the masses of the ions in the sample, the spectrum of the masses can be determined from the measured times of travel.
The accuracy of the flight time measurement, and hence the mass measurement, is improved if the initial pulse of secondary ions is made shorter in duration. Specifically the uncertainty in flight time measurement is always greater or equal to the duration of the primary excitation ion pulse at the sample surface.
For the charged particle source to produce the necessary short bursts of charged particles, the source is gated, that is it is switched on and off very quickly. The ratio of on-time divided by off-time of the particle source is referred to as the duty cycle of the source, and is typically less than one in one thousand when the source is used in a time of flight mass spectrometer. The average particle current is equal to the current produced by the source when switched on, multiplied by the duty cycle, and normally this average current limits the rate at which useful data can be collected from the spectrometer. The duty cycle at the sample cannot be increased without a loss in the relative accuracy of the time measurement, so it is therefore desirable to start with a relatively long burst from the ion source and bunch it in such a way that the number of particles in the burst remains constant, but the duration of the burst as it hits the sample surface is much shorter.
If one considers a pulse of particles all travelling at the same velocity, but spread out in space, in order to bunch the particles and thus cause all the particles to hit the sample within a very short duration, it is necessary to impose a small velocity spread in the particles in such a manner as to cause particles at the tail of the pulse to catch up with those at the front of the pulse during the time taken for the pulse to travel to the sample.
2. Description of the Prior Art
One conventional ion source will now be described with reference to FIG. 1, which is a schematic diagram of an ion source buncher for the ion source, and a sample.
Referring to the figure, an ion gun 1 is arranged to provide a continuous constant particle beam with energy provided by the high voltage supply 2 of, for example, 25 kilo electron volts. From this beam a primary pulse 3, may be chopped by causing the beam to be scanned across an aperture 4, by means of deflection plates 5, commonly referred to as blanking plates. It will be appreciated, however, that other arrangements can give the same result, for example an electrostatic sector energy filter using pulsed excitations as used by Benninghoven.
Bunching of the ions in the pulses 3 is produced by a buncher 6. This consists of a parallel plate capacitor 7,8, with a hole 9 through the centre through which ions may pass. An instantaneous voltage edge 10 is applied to either plate of the capacitor 7,8, whilst the primary pulse 3 is between the plates 7,8, in such a way as to accelerate the ions in the direction of the sample 11. For example if the ions are positive ions, a positive voltage edge, indicated as 10, could be applied to the plate 7. As ions at the tail end of the pulse will receive a greater energy impulse than those at the leading edge of the pulse, there will be a first order correction in the time taken for the ions to travel to the sample, and thus a bunching effect of the ions within each pulse will occur. The energy dispersion required, and hence the magnitude of the voltages required to be applied to the plates 7,8, depends on the distance l.sub.s from the plates 7,8 to the sample 11 and the initial energy V.sub.o of the primary pulses.
The voltage edge V.sub.b required to be applied to the plate 7 may be expressed by: EQU V.sub.b =2V.sub.o l.sub.b /l.sub.s
where l.sub.b is the distance between the plates 7,8 in the buncher 6.
For the example of pulses of Gallium ions of V.sub.o =25 kV starting energy and 20 nanoseconds duration, the primary pulses will be 5.4 mm in length. If the distance l.sub.s from the plates 7,8 to the sample 11 is 80 mm, the energy spread required is 3.4 kV. Thus if the distance l.sub.b between the plates 7,8 is chosen to be 8 mm so as to comfortably accommodate each unbunched primary pulse, this will necessitate a 5 kV voltage edge with a rise time of about 2 nanoseconds.
While such an arrangement is relatively satisfactory, the charged particle source suffers the disadvantages that it is necessary to incorporate and align extra hardware constituting the buncher 6. Furthermore, it is difficult to arrange for l.sub.s to be very large so as to reduce the necessary bunching voltage V.sub.b, as the blanking plates and aperture 4,5 as well as ion optics (not shown) must be arranged between the source and the buncher 6. Also if the ion beam is to be focussed, placing the source further from the sample 11 will result in a poorer focal spot. Furthermore, the slew rate of the power supply (not shown) for the buncher 6 has to be extremely fast as the full voltage has to be reached whilst each ion pulse 3 is contained between the plates 7,8 and the timing of the edge 10 is critical to the region of nanoseconds. Whilst suitable pulsed power supplies do exist, they are very expensive and have repetition rate and lifetime limitations.
In order to reduce the necessary voltage edge V.sub.b, it is possible to form the buncher 9 of a number of stages, each of the capacitor form described above. Whilst this has the advantage that the magnitude of the voltage edge V.sub.b required is reduced proportionately by the number of stages, the arrangement has the disadvantages that extra hardware is required in the ion source, that is one plate for each stage, these plates being difficult to align. Furthermore the slew rate of the power supply required is still very high.